Technical Field
This invention generally relates to scanning mechanisms for digital imaging scanners, facsimile machines and the like. More particularly, this invention concerns a method and an apparatus for scanning at multiple vertical resolutions, using the same scanning mechanism, with relatively small, or limited, changes in the vertical scanning velocity.
Background
Digital imaging has seen a proliferation of resolution standards. Currently there are three resolution standards which have gained wide acceptance in the industry, which is due mainly to their acceptance in the market place. Resolutions of 204.times.98 dots per inch (DPI) and 204.times.196 DPI have become the standards in group 3 facsimile transmission devices while a resolution of 300.times.300 DPI is the current standard for document printing and desktop publishing. Newer printing technologies should result in higher resolutions such as 600.times.600 DPI, 900.times.900 DPI or even 1200.times.1200 DPI becoming standard.
Unfortunately, as desirable as it may be, it is difficult to design a scanning device which is capable of scanning at all of the various resolutions and to thereby enable the consumer to purchase a single device which will satisfy all scanning needs, e.g. capable of scanning at 204.times.98 DPI or 204.times.196 DPI for sending a fax, or at 300.times.300 DPI or 600.times.600 DPI for importing graphics into computer generated documents.
To date the solution to the problem has been to scan an image at a given resolution and then "process" the image to convert it to the desired resolution and/or size. Digital image processing, as the phrase is used here, refers to the process of mathematically assigning a value to each pixel of the image representing its tone or greyscale value, and then either averaging groups of pixels to obtain a "super" pixel, or conversely, letting each pixel represent a "super" pixel, which is subsequently translated into a larger group of pixels, depending upon whether the conversion is to either a lower or higher resolution, respectively. Additional processing techniques can be used to refine the image using various algorithms, which are well known.
Unfortunately, even simple digital processing techniques require tremendous amounts of processor time and computing capacity. This constraint has prevented "real time" faxing from high resolution scanners, i.e. where the documents are scanned as the data is being communicated. Instead, all of the pages of a particular transmission are first scanned into the memory of the device, converted to the requisite resolution and stored, prior to transmission. Once this preparation is complete, the processor can focus on the transmission process. The use of a coprocessor to handle the resolution conversion has proven to be both too expensive and complicated.
Another proposed solution, at least to the vertical resolution conversion problem, was to design a mechanism whereby the scanner itself would traverse the paper at variable speeds depending upon the resolution chosen. This would be accomplished by either a variable speed paper advance mechanism, similar to those used in a typical fax machine where the paper moves and the scanner bar remains stationary, or by a variable speed scanner transport mechanism where the paper stays stationary and the scanner bar traverses the paper. This solution would use a stepper motor which could be run at any speed across a range of necessary speeds. In order to accomplish this, the motor would have to be selected for the lowest resolution, since this corresponds to the highest paper velocity or scan bar velocity, hereinafter simply be referred to as paper velocity. To function properly, the mechanism would have to be tuned, at each particular velocity, to prevent unwanted vibration. Due to the expense of the motor, problems of paper drag, mechanism vibration and fundamental frequency harmonics, the design is both too expensive and complex.
What is needed is a method to scan at multiple resolutions which does not require significant processing capability, does not require a complex paper advance or scanner advance mechanism, and which is inexpensive and relatively simple to implement.